I still believe whatever you're experiencing is related to the usescancodes option in DOSBox Prefs.txt

Indeed, it seems that older builds had usescancodes==false, while this build has usescancodes==true by default. In my short experience I can map all N810 keys only if it's true. Having it set to false causes problems like the one someone describe a few pages ago (the DOSBox mapper accepting mappings for a single session then "forgetting" about them instead of saving them), but, as I said, this is based on my experience only. So it may work better with usescancodes==false on N800 or whatever.